Solid-state image capturing apparatus, driving method thereof and electronic apparatus

ABSTRACT

A solid-state image capturing apparatus including: a pixel array unit including two-dimensionally arranged pixels each including a photoelectric conversion unit, a transfer transistor that transfers charges accumulated in the photoelectric conversion unit, and a charge discharging transistor that selectively discharges the charges accumulated in the photoelectric conversion unit; and a driving unit that performs driving for reading signals from each pixel of the pixel array unit, and drives the charge discharging transistor by using a signal for the driving.

RELATED APPLICATION DATA

This application is a continuation of U.S. patent application Ser. No.12/690,557, filed on Jan. 20, 2010, the entirety of which isincorporated herein by reference to the extent permitted by law. Thepresent invention claims priority to and contains subject matter relatedto Japanese Patent Application No. JP 2009-016266 filed in the JapanPatent Office on Jan. 28, 2009, the entire contents of which beingincorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates to a solid-state image capturingapparatus, a driving method thereof and an electronic apparatus. Inparticular, the invention relates to a solid-state image capturingapparatus, such as a CMOS image sensor, having an electronic shutterfunction, a driving method thereof, and an electronic apparatus havingthe same.

2. Description of the Related Art

Most CMOS image sensors, which are a kind of a solid-state imagecapturing apparatus, for example, an X-Y address type solid-state imagecapturing apparatus, have an electronic shutter function forelectronically performing an operation of resetting signal chargesaccumulated in photoelectric converters (light receiving units) andnewly starting accumulation of signal charges. In general, a CMOS imagesensor employs a so-called rolling shutter (also referred to as a focalplane shutter) scheme for performing the starting and ending of exposurewith respect to each pixel low through sequential scanning.

In the case of employing a rolling shutter scheme for an electronicshutter, since an exposure period is not synchronized with respect toeach pixel row, if an object being moved or an object being turned onand off is photographed, an image may be distorted or broken. In thisregard, global exposure is realized to perform the starting and endingof exposure at the same timing with respect to all pixels, so thatdistortion can be prevented when the object being moved or the objectbeing turned on and off is photographed.

Therefore, in order to realize the global exposure by using a CMOS imagesensor, a configuration is employed, in which a holding unit (FD or CCD)for holding signal charges is provided separately from a photoelectricconversion unit. Further, the signal charges are transferred from thephotoelectric conversion unit to the holding unit with respect to allpixels, so that the exposure end is simultaneously performed withrespect to all pixels and synchronization of the exposure period isrealized.

However, in a CMOS image sensor employing the configuration in which theholding unit of signal charges is added, during reading of signalcharges (photoelectric charges) after exposure, that is, during theperiod for which signal charges are continuously held in the holdingunit, accumulation of electric charges occurs again because thephotoelectric conversion unit continues to receive incident light. Thus,when the incident light is strong, the photoelectric conversion unit maybe saturated again. Further, if signal charges overflowing from thephotoelectric conversion unit leak in the holding unit, an image may bebroken.

Therefore, in the related art, separately from a transfer transistorthat transfers signal charges in the photoelectric conversion unit to anFD (floating diffusion) section, a charge discharging transistor, whichdischarges the signal charges in the photoelectric conversion unit to apart different from the FD section, is provided inside the pixel (e.g.,refer to Japanese Unexamined Patent Application Publication No.2004-140149).

SUMMARY OF THE INVENTION

However, if a configuration, in which the charge discharging transistoris newly added to the pixel, is employed, since the number of signalinterconnections for driving the charge discharging transistor isincreased, layout efficiency is reduced. Further, since the number ofinterconnections of interconnection layers of an upper layer of thephotoelectric conversion unit as the increase in the number of thesignal interconnections, if light incident to the photoelectricconversion unit is rejected by upper layer interconnections, the amountof light incident into the photoelectric conversion unit is reduced, sothat sensitivity is reduced.

In view of the above issues, it is desirable to provide a solid-stateimage capturing apparatus, which can prevent layout efficiency andsensitivity from being reduced even in the case of employing a pixelstructure having a charge discharging transistor, a driving methodthereof, and an electronic apparatus having the same.

According to an embodiment of the invention, there is provided asolid-state image capturing apparatus including a pixel array unitincluding two-dimensionally arranged pixels each including aphotoelectric conversion unit, and a charge discharging transistor thatselectively discharges charges accumulated in the photoelectricconversion unit, wherein the charge discharging transistor is drivenusing a signal for performing driving for reading signals from eachpixel of the pixel array unit.

In the solid-state image capturing apparatus having the aboveconfiguration, the fact that the charge discharging transistor is drivenusing the driving signal when performing the driving for reading thesignals from each pixel means that the driving signal for the signalreading is commonly used as a driving signal of the charge dischargingtransistor. Thus, even if a signal line for transmitting the drivingsignal of the charge discharging transistor is not provided, the chargedischarging transistor can be driven using a signal line fortransmitting the driving signal for the signal reading. That is, in thecase of employing a pixel structure including the charge dischargingtransistor that selectively discharges the charges accumulated in thephotoelectric conversion unit, even if a signal line dedicated fordriving the charge discharging transistor is not added, the chargedischarging transistor can be driven.

According to the invention, in the pixel structure provided with thecharge discharging transistor, the charge discharging transistor can bedriven even without adding a dedicated signal line, so that reduction inthe layout efficiency or sensitivity accompanied by the addition of thecharge discharging transistor is not caused.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a system configuration diagram illustrating an outline of theconfiguration of a CMOS image sensor to which the invention is applied;

FIG. 2 is a circuit diagram illustrating the circuit configuration of acircuit example 1 of a pixel structure which has no memory unit insidethe pixel;

FIG. 3 is a circuit diagram illustrating the circuit configuration of acircuit example 2 of a pixel structure which has a memory unit insidethe pixel;

FIG. 4 is a circuit diagram illustrating the configuration of mainelements of a CMOS image sensor according to a first embodiment of theinvention;

FIG. 5 is a timing chart illustrating one example of a driving method ofa CMOS image sensor according to a first embodiment;

FIG. 6 is a timing chart illustrating a driving example when a rollingshutter operation is performed according to a first embodiment;

FIG. 7 is a pixel layout diagram illustrating one example of arrangementof transistors and lines;

FIG. 8 is a circuit diagram illustrating the configuration of mainelements of a CMOS image sensor according to a second embodiment of theinvention;

FIG. 9 is a circuit diagram illustrating the configuration of mainelements of a CMOS image sensor according to a third embodiment of theinvention;

FIG. 10 is a timing chart illustrating one example of a driving methodof a CMOS image sensor according to a third embodiment;

FIG. 11 is a timing chart illustrating a driving example when a rollingshutter operation is performed according to a third embodiment;

FIG. 12 is a circuit diagram illustrating the configuration of mainelements of the CMOS image sensor according to a fourth embodiment ofthe invention;

FIG. 13 is a circuit diagram illustrating the configuration of mainelements of the CMOS image sensor according to a fifth embodiment of theinvention;

FIG. 14 is a potential diagram illustrating a design example when it isnecessary to extract charges accumulated in a photodiode in a state inwhich photoelectric charges are held in a capacitance Cfd and acapacitance Cm;

FIG. 15 is a potential diagram illustrating an example in which chargesare transferred to a capacitance Cfd without discharging chargesaccumulated in a photodiode;

FIGS. 16A and 16B are diagrams illustrating a stats in which chargedischarging is completed without addition of excess charges to retentioncharges held in a capacitance Cfd;

FIG. 17 is a circuit diagram illustrating the configuration of mainelements when the invention is applied to a pixel structure for holdingby two horizontal pixels; and

FIG. 18 is a block diagram illustrating a configuration example of animage capturing apparatus which is one example of an electronicapparatus according to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, preferred embodiments of the invention (hereinafter,referred to as embodiments) will be described in detail with referenceto the accompanying drawings. Furthermore, the description will be givenin order of the following items.

1. Solid-state image capturing apparatus (example of a CMOS imagesensor) to which the invention is applied

1-1. System configuration

1-2. Circuit configuration of a unit pixel

2. First embodiment (example in which a selection pulse SEL is commonlyused)

3. Second embodiment (first example in which a reset pulse RST iscommonly used)

4. Third embodiment (second example in which a reset pulse RST iscommonly used)

5. Fourth embodiment (example in which a transmission pulse TRG iscommonly used)

6. Fifth embodiment (example in which a transmission pulse TRX iscommonly used)

7. Application (example of a holding structure by plural pixels)

8. Modification

9. Electronic apparatus (example of image capturing apparatus)

1. Solid-State Image Capturing Apparatus to which the Invention isApplied

[1-1. System Configuration]

FIG. 1 is a system configuration diagram illustrating an outline of theconfiguration of a CMOS image sensor which is a kind of a solid-stateimage capturing apparatus to which the invention is applied, forexample, an X-Y address type solid-state image capturing apparatus.

As shown in FIG. 1, the CMOS image sensor 10 according to an applicationof the invention includes a pixel array unit 12 formed on asemiconductor substrate (chip) 11, and a peripheral circuit portionintegrated on the same chip 11 as the pixel array unit 12. As theperipheral circuit, for example, a vertical driving unit 13, a columnprocessing unit 14, a horizontal driving unit 15, an output circuit unit16 and a system control unit 17 are provided.

In the pixel array unit 12, unit pixels (not shown, which are sometimeswritten as merely “pixels” in the following description) aretwo-dimensionally arranged in a matrix, each of which includes aphotoelectric conversion unit (photoelectric conversion element) thatphotoelectrically converts incident visible rays and accumulates signalcharges (photoelectric charges) of charge amount corresponding to thelight amount thereof. The detailed configuration of the unit pixel willbe described later.

Further, in the pixel array unit 12, pixel driving lines 121 arearranged with respect to each row of matrix-type pixel arrangement alongthe left and right directions (pixel arrangement direction/horizontaldirection of pixel rows) of FIG. 1, and vertical signal lines 122 arearranged with respect to each column thereof along the up and downdirections (pixel arrangement direction/vertical direction of pixelcolumns) of FIG. 1. In FIG. 1, each pixel driving line 121 is shown asone line, but the invention is not limited thereto. One end of eachpixel driving line 121 is connected to an output terminal correspondingto each row of the vertical driving unit 13.

The vertical driving unit 13 is configured by a row scanning circuitincluding a shift register and an address decoder, and denotes a pixeldriving unit that drives all pixels of the pixel array unit 12 at thesame time in units of row or the like. A detailed configuration of thevertical driving unit 13 is not shown, but, in general, the verticaldriving unit 13 includes two scanning systems, that is, a readingscanning system and a sweep scanning system.

The reading scanning system selectively scans unit pixels of the pixelarray unit 12 sequentially for reading signals from the unit pixels. Thesweep scanning system performs sweep scanning with respect to a readrow, which is scanned by the reading scanning system before the readingscanning by the time of shutter speed.

According to the sweeping scanning by the sweep scanning system,unnecessary charges are swept out (reset) from the photoelectricconversion units of the unit pixels of the read row. Then, a so calledelectronic shutter operation is performed by the sweeping (reset) of theunnecessary charges by the sweep scanning system. Herein, the electronicshutter operation means an operation in which photoelectric charges ofthe photoelectric conversion elements are swept off and exposure isnewly started (accumulation of signal charges is started).

Signals read by the reading operation performed by the reading scanningsystem correspond to the amount of light incident after the readingoperation just before that or the electronic shutter operation. Further,the period from the reading timing by the reading operation just beforeor the sweeping timing by the electronic shutter operation to thereading timing by the reading operation at this time will beaccumulation time (exposure time) of photoelectric charges in the unitpixel.

Signals output from each unit pixel of the pixel row having beenselectively scanned by the vertical driving unit 13 are supplied to thecolumn processing unit 14 through the respective vertical signal lines122. The column processing unit 14 performs a predetermined signalprocessing with respect to signals output from each unit pixel of aselected row through the vertical signal lines 122 by each pixel columnof the pixel array unit 12, and temporarily holds pixel signals afterthe signal processing.

In detail, the column processing unit 14 receives signals of each unitpixel and, for example, performs the signal processing, such as noiseelimination by CDS (Correlated Double Sampling), signal amplification orAD (Analog-Digital), with respect to the signals. Fixed pattern noise(e.g., reset noise or variation of a threshold value of an amplifyingtransistor), which is peculiar to pixels, is eliminated by the noiseelimination process. The signal processing exemplified here is justexamples, and is not limited thereto.

The horizontal driving unit 15 is configured by a column scanningcircuit including a shift register and an address decoder, andsequentially selects unit circuits corresponding to pixel columns in thecolumn processing unit 14. According to the selective scanning by thehorizontal driving unit 15, pixel signals processed in the columnprocessing unit 14 by each unit circuit are sequentially output to ahorizontal bus 18 and transmitted to the output circuit unit 16 throughthe horizontal bus 18.

The output circuit unit 16 processes and outputs the signals transmittedthrough the horizontal bus 18. As the processing in the output circuitunit 16, in addition to a case in which only a buffering processing isperformed, various digital signal processing such as adjustment of ablack level before the buffering or correction of variation by eachcolumn may be cited.

The system control unit 17 receives a clock supplied from the outside ofthe chip 11, data indicating an operation mode or the like, and outputsdata such as internal information of the CMOS image sensor 10. Further,the system control unit 17 includes a timing generator that generatesvarious timing signals, and controls the driving of the peripheralcircuit portion such as the vertical driving unit 13, the columnprocessing unit 14 and the horizontal driving unit 15 based on thetiming signals generated by the timing generator.

[1-2. Circuit Configuration of the Unit Pixel]

Next, a detailed example of the circuit configuration of the unit pixel20 will be described. Herein, two circuit configurations of a case of apixel structure, which has no memory unit that holds charges inside thepixel, and a case of a pixel structure, which has the memory unit, willbe described as an example. Hereinafter, a circuit example of the pixelstructure, which has no memory unit, will be referred to as a circuitexample 1 and a circuit example of the pixel structure, which has thememory unit, will be referred to as a circuit example 2.

(Example of the Pixel Structure which has No Memory Unit Inside thePixel)

FIG. 2 is a circuit diagram illustrating the circuit configuration ofthe circuit example 1 of the pixel structure which has no memory unitinside the pixel. The unit pixel 20A according to the circuit example 1,for example, includes a transfer transistor 22, a reset transistor 23,an amplifying transistor 24, a select transistor 25 and a chargedischarging transistor 26, in addition to a photodiode 21 which is aphotoelectric conversion unit.

The photodiode 21 has an anode electrode connected to a negative powersource (e.g., ground), photoelectrically converts received visible raysinto photoelectric charges (herein, photoelectrons), whose amount isproportional to the light amount thereof, and accumulates thephotoelectric charges. The photodiode 21 has a cathode electrodeelectrically connected to a gate electrode of the amplifying transistor24 via the transfer transistor 22.

Hereinafter, a node 27 electrically connected to the gate electrode ofthe amplifying transistor 24 will be referred to as an FD (FloatingDiffusion/Floating Diffusion Region) section. That is, the FD section 27is a node including an interconnection through which a diffusion layercorresponding to a drain region of the transfer transistor 22 isconnected to the gate electrode of the amplifying transistor 24, andincludes a floating capacitance Cfd.

The transfer transistor 22 is connected between the cathode electrode ofthe photodiode 21 and the FD section 27. The transfer transistor 22 isturned on when a transmission pulse TRG is supplied to a gate electrodethereof, so that photoelectric charges accumulated in the photodiode 21are transferred to the FD section 27.

The reset transistor 23 includes the FD section 27 serving as a firstmain electrode and a second main electrode connected to a power line ofa supply voltage Vdd. In the example, the first main electrode is asource electrode and the second main electrode is a drain electrode. Thereset transistor 23 is turned on when a reset pulse RST is supplied to agate electrode thereof, so that holding charges of the floatingcapacitance Cfd are discarded to the supply voltage Vdd and thus thefloating capacitance Cfd is reset. The FD section 27, that is, if thefloating capacitance Cfd is reset, the unit pixel 20 is reset.

The amplifying transistor 24 has the gate electrode connected to the FDsection 27 and a drain electrode connected to the power line of thesupply voltage Vdd, and outputs a signal having a level corresponding tothe potential of the FD section 27. The select transistor 25, forexample, has a drain electrode connected to a source electrode of theamplifying transistor 24, and a source electrode connected to thevertical signal line 122.

The select transistor 25 is turned on when a selection pulse SEL issupplied to a gate electrode thereof, so that the unit pixel 20 enters aselection state and a signal supplied from the amplifying transistor 24is output to the vertical signal line 122. With regard to the selecttransistor 25, a circuit configuration connected between the power lineof the supply voltage Vdd and the drain electrode of the amplifyingtransistor 24 may be employed.

The charge discharging transistor 26 is connected between the photodiode21 and a charge discharging node (e.g., the power line of the supplyvoltage Vdd). The charge discharging transistor 26 is turned on when acharge discharging pulse OFG is supplied to a gate electrode thereof, sothat the photoelectric charges in the photodiode 21 are selectivelydiscarded to the charge discharging node.

The charge discharging transistor 26 is provided in the followingpurpose. That is, as described above, since the charge dischargingtransistor 26 is turned on during the period for which the accumulationof photoelectric charges is not performed, it can be prevented thatexcessive light is incident into the photodiode 21 and saturated withphotoelectric charges, and then charges exceeding the saturation chargeamount is overflowed to the FD section 27 or the periphery thereof.

In the unit pixel 20A according to the circuit example 1 having theabove configuration, after the transfer transistor 22 or the chargedischarging transistor 26 is turned on, the charges accumulated in thephotodiode 21 are discharged, and then the transfer transistor 22 or thecharge discharging transistor 26 is turned on. In this way, theaccumulation of photoelectric charges in the photodiode 21 is started.

Upon the exposure end, the transfer transistor 22 is turned on, so thatthe photoelectric charges accumulated in the photodiode 21 aretransferred to the floating capacitance Cfd of the FD section 27. Beforethe photoelectric charges are transferred, the FD section 27 is reset inadvance by turning on the reset transistor 23. Then, the voltage of theFD section 27 directly after the reset is read as a reset level by theamplifying transistor 24, and is output to the vertical signal line 122through the select transistor 25.

After the reset level is read, the charges are transferred to the FDsection 27, the voltage of the FD section 27 at that time is read as asignal level by the amplifying transistor 24, and is output to thevertical signal line 122 through the select transistor 25. The sequenceof a signal read by the amplifying transistor 24 is not limited to thesequence of the reset level and the signal level. That is, after thephotoelectric charges are transferred to the FD section 27 from thephotodiode 21, the signal level may be read. Thereafter, the FD section27 is reset and the reset level may be read.

In this way, the reset level and the signal level sequentially outputfrom the unit pixel 20A to the vertical signal line 122 is supplied tothe column processing unit 14 (see FIG. 1), which will be describedlater, through the vertical signal line 122. Then, in the columnprocessing unit 14, a CDS processing is performed to obtain thedifference between the reset level and the signal level, so that theoriginal signal (correct signal), from which fixed pattern noisepeculiar to pixels are eliminated, is obtained.

In the unit pixel 20A according to the circuit example 1, thetransmission pulse TRG, the reset pulse RST and the selection pulse SELserve as driving signals used for performing driving for reading signalsfrom the unit pixel 20A.

(Example of the Pixel Structure which has the Memory Unit Inside thePixel)

FIG. 3 is a circuit diagram illustrating the circuit configuration ofthe circuit example 2 of the pixel structure which has the memory unitinside the pixel. In FIG. 3, the same reference numerals are used todesignate the same elements as those shown in FIG. 2.

The unit pixel 20B according to the circuit example 2 includes a memoryunit 28 in addition to the photodiode 21, the transfer transistor 22,the reset transistor 23, the amplifying transistor 24, the selecttransistor 25 and the charge discharging transistor 26. The memory unit28 is provided between the photodiode 21 and the transfer transistor 22.

Since the connection relationships and functions of the photodiode 21,the transfer transistor 22, the reset transistor 23, the amplifyingtransistor 24, the select transistor 25 and the charge dischargingtransistor 26 are basically identical to those of the elements accordingto the unit pixel 20A according to the circuit example 1, detaileddescription thereof will be omitted in order to avoid the redundancy.The memory unit 28 includes a capacitance Cm for holding photoelectriccharges, and has a transfer function of transferring photoelectriccharges from the photodiode 21 to the capacitance Cm in response to atransmission pulse TRX.

As the pixel structure having the memory unit 28, for example, anembedded channel structure, which is similar to that of a CCD (ChargeCoupled Device), can be employed. However, the invention is not limitedto the embedded channel structure, and well known structures (e.g.,refer to Japanese Unexamined Patent Application Publication No.2006-311515 or Japanese Unexamined Patent Application Publication No.2007-503722) may be employed in addition to the embedded channelstructure.

The original purpose of providing the memory unit 28 inside the pixellies in realizing a noise reduction processing including kTC noise inthe reset operation of pixels (e.g., refer to Japanese Unexamined PatentApplication Publication No. 2006-311515 or Japanese Unexamined PatentApplication Publication No. 2007-503722). Further, if the pixelstructure, in which the memory unit 28 capable of holding charges insidethe pixel is provided in addition to the FD section 27, is employed, itis possible to obtain the effect of preventing overflowing ofphotoelectric charges upon the saturation.

In the unit pixel 20B according to the circuit example 2, thetransmission pulse TRG, the reset pulse RST, the selection pulse SEL andthe transmission pulse TRX serve as driving signals used for performingdriving for reading signals from the unit pixel 20B.

As described above, in the CMOS image sensor 10, when employing thepixel structure in which the charge discharging transistor 26 isprovided in the unit pixel 20 (20A and 20B), the essence of theinvention lies in that the layout efficiency or the sensitivity isprevented from being reduced. Hereinafter, detailed embodiments thereofwill be described.

2. First Embodiment

[System Configuration]

FIG. 4 is a circuit diagram illustrating the configuration of mainelements of the CMOS image sensor according to the first embodiment ofthe invention. Herein, the CMOS image sensor, in which the unit pixel20A of the circuit example 1 shown in FIG. 2 is two-dimensionallyarranged, is described as an example. Further, in order to simplify FIG.4, two unit pixels 20A-i and 20A-i-1 of a certain pixel column, whichare vertically adjacent to each other, are representatively shown.

As shown in FIG. 4, in the unit pixels 20A-i and 20A-i-1, as the pixeldriving lines 121, for example, three driving lines such as atransmission line 121-1, a reset line 121-2 and a selection line 121-3are provided to each pixel row. These driving lines 121-1, 121-2 and121-3 are respectively connected to output terminals corresponding tothe vertical driving unit 13 by each pixel row.

Then, description will be given while focusing on an i^(th) pixel row.The transmission line 121-1 supplies the gate electrode of the transfertransistor 22 with a transmission pulse TRGi of an i^(th) row, which isoutput from the vertical driving unit 13, in units of pixel row. Thereset line 121-2 supplies the gate electrode of the reset transistor 23with a reset pulse RSTi of the i^(th) row, which is output from thevertical driving unit 13, in units of pixel row.

The selection line 121-3 supplies the gate electrode of the selecttransistor 25 with a selection pulse SELi of the i^(th) row, which isoutput from the vertical driving unit 13, in units of pixel row, andsupplies the gate electrode of the charge discharging transistor 26 ofthe previous row (i.e., an (i−1)^(th) row) with the i^(th) selectionpulse SELi in units of pixel row. That is, as the charge dischargingpulse OFGi-1 of the charge discharging transistor 26 of the unit pixel20A-i-1 for which signal reading is previously performed with respect toa row scanning direction, the selection pulse SELi of the selecttransistor 25 of the unit pixel 20A-i, for which signal reading isperformed thereafter, is commonly used.

[Driving Example]

Next, one example of a driving method of the CMOS image sensor accordingto the first embodiment will be described using the timing chart of FIG.5.

First, with respect to all pixel rows, the transmission pulse TRGi andthe reset pulse RSTi are in an active state (high level) at time t11,and charges accumulated in the photodiode 21 are discharged, so that theglobal exposure is started. With respect to the operation of dischargingthe accumulated charges, since no problem may occur although the chargedischarging transistor 26 is turned on as described above, the selectionpulse SEL may be in the active state in the global exposure.

During the period for which the global exposure is performed,photoelectric charges are accumulated in the photodiode 21. Then, thetransmission pulse TRGi is in the active state at time t12 at which theglobal exposure is ended, so that the charges accumulated in thephotodiode 21 are transferred to the floating capacitance Cfd of the FDsection 27. The above operation is performed during the period for whichthe global exposure is performed.

Reading of signals from the unit pixel 20A is performed by each pixelrow. In detail, in signal reading of the (i−1)^(th) row in which theselection pulse SELi-1 is in the active state at time t13, first, thephotoelectric charges held in the floating capacitance Cfd of the FDsection 27 are read as a signal level. The period for which the signallevel is read will be referred to as D period.

Next, the reset pulse RSTi is in the active state at time t14, so thatthe floating capacitance Cfd of the FD section 27 is reset. Then, thevoltage of the FD section 27 when the floating capacitance Cfd is resetis read as a reset level. The period for which the reset level is readwill be referred to as P period.

Consecutively, the reason for entering the reading operation of thei^(th) row will be described. An operation is performed between thereading operation of the (i−1)^(th) row and the reading operation of thei^(th) row to prevent the photoelectric charges from overflowing due tothe saturation of the photodiode 21. In detail, in the period betweent15 and t16, the selection pulse SELi is in the active state, and thecharge discharging transistor 26 is turned on, so that the photoelectriccharges of the photodiode 21 are charged to the supply voltage Vdd.Thus, the saturation of the photodiode 21 can be prevented.

As described above, in the CMOS image sensor according to the firstembodiment, the charge discharging transistor 26 is driven using theselection pulse SELi when performing the driving for reading the signalsfrom the unit pixel 20A-i. Further, since the selection pulse SELi isused as the charge discharging pulse OFG of the charge dischargingtransistor 26, even if a signal line dedicated for the chargedischarging pulse OFG is not provided, the selection line 121-3 for theselection pulse SEL is used as the dedicated signal line, so that thecharge discharging transistor 26 can be driven.

That is, in the CMOS image sensor having the pixel structure with thecharge discharging transistor 26, even if the signal line dedicated fordriving the charge discharging transistor 26 is not added, the chargedischarging transistor 26 can be driven. In this way, the configuration,in which the signal line dedicated for driving the charge dischargingtransistor 26 is not necessary, is employed, so that the reduction inthe layout efficiency or sensitivity accompanied by the addition of thecharge discharging transistor 26 is not caused.

Further, according to the embodiment, as the driving signal (i.e., thecharge discharging pulse OFG) of the charge discharging transistor 26 ofthe (i−1)^(th) row, the selection pulse SELi of the select transistor 25of the i^(th) row for which the signal reading is performed after the(i−1)^(th) row is used. However, the invention is not limited thereto.That is, on the assumption that the exposure period is separated fromthe reading period by only the global exposure operation, no problemoccurs even if the selection pulse SEL of an arbitrary pixel row isused.

In the common use of the selection pulse SEL, as in the case of thedriving example of FIG. 5, as the charge discharging pulse OFG of thecharge discharging transistor 26 in a pixel previously read with respectto the row scanning direction (reading scanning direction), it ispreferred that the selection pulse SEL in a pixel, which is readthereafter, is used. The reason for this is as follows.

As the charge discharging pulse OFG in the pixel previously read withrespect to the row scanning direction, it is preferred that theselection pulse SEL of the pixel, which is read thereafter, is used, sothat it is possible to deal with the rolling shutter operation which isgeneral driving of the CMOS image sensor. Herein, differently from theglobal exposure, the rolling shutter operation corresponds to a drivingmethod in which exposure is started after the charges accumulated in thephotodiode 21 are sequentially discharged in the row scanning direction,and the end of the exposure and the reading of signals are sequentiallyperformed.

FIG. 6 illustrates a driving example in the case of the rolling shutteroperation. When focusing on the (i−1)^(th) row, exposure is startedafter the charges accumulated in the photodiode 21 are discharged duringthe reading period of a different row (an (n−1)^(th) row), and then thereset level in the P period and the signal in the D period during thereading period of the (i−1)^(th) row are respectively read. Similarly tothis, in relation to the next i^(th) row, exposure is started in adifferent row (n^(th) row).

In such a case, as the charge discharging pulse OFG in the pixelpreviously read, the selection pulse SEL of the pixel, which is readthereafter, is used, so that it is possible to drive the CMOS imagesensor such that the accumulated charges before reading can be preventedfrom being discarded from the charge discharging transistor 26 by theselection pulse SEL of a different row.

As described above, the charge discharging pulse OFG held in common issatisfied if the charge discharging pulse OFG is advanced with respectto the selection pulse SEL by one row (one pixel). Thus, the number ofadvanced rows is arbitrary. In addition, when the charge dischargingpulse OFG is held in common in a pixel advanced by two rows (two pixels)or more, an efficient reading sequence may be obtained.

For example, a case may occur in which plural types of color filterssuch as Bayer arrangement are stacked in each pixel, and, in anoperation of adding pixels having the same color, the pixels having thesame color are continuously read by two rows. That is, an operationsequence, in which an (i+2)^(th) row is read next to an i^(th) row andthen an (i+1)^(th) row and an (i+3)^(th) row are read, may be efficient.In such a case, it is preferred that the selection pulse SEL of a pixeladvanced by two rows is used as the charge discharging pulse OFG.

FIG. 7 is one example of arrangement of transistors and lines. In suchan example, by the selection line 121-3 of the selection pulse SEL of ani^(th) row advanced by two rows, the gate electrode of the chargedischarging transistor 26 of an (i−2)^(th) row is driven. In FIG. 7, inorder to simplify the drawing, the pixel driving line 121 other than theselection line 121-3, that is, the transmission line 121-1 and the resetline 121-2 are not shown.

Further, according to the embodiment, a case in which the unit pixel 20is the unit pixel 20A of the circuit example 1 as shown in FIG. 2 isdescribed as an example. However, no problem occurs even if the unitpixel 20 is the unit pixel 20B of the circuit example 2 as shown in FIG.3, that is, the unit pixel 20B having the memory unit 28 separately fromthe FD section 27.

3. Second Embodiment

FIG. 8 is a circuit diagram illustrating the configuration of mainelements of the CMOS image sensor according to the second embodiment ofthe invention. In FIG. 8, the same reference numerals are used todesignate the same elements as those shown in FIG. 4.

The CMOS image sensor according to the embodiment is based on the CMOSimage sensor in which the unit pixel 20A of the circuit example 1 shownin FIG. 2 is two-dimensionally arranged. Further, in order to simplifyFIG. 8, two unit pixels 20A-i and 20A-i-1 of a certain pixel column,which are vertically adjacent to each other, are representatively shown.

As shown in FIG. 8, in the unit pixels 20A-i and 20A-i-1, as the pixeldriving lines 121, for example, three driving lines such as thetransmission line 121-1, the reset line 121-2 and the selection line121-3 are provided by each pixel row. These driving lines 121-1, 121-2and 121-3 are respectively connected to output terminals correspondingto the vertical driving unit 13 by each pixel row.

Then, description will be given while focusing on the i^(th) pixel row.The transmission line 121-1 supplies the gate electrode of the transfertransistor 22 with the transmission pulse TRGi of the i^(th) row, whichis output from the vertical driving unit 13, in units of pixel row. Theselection line 121-3 supplies the gate electrode of the selecttransistor 25 with the selection pulse SELi of an i^(th) row, which isoutput from the vertical driving unit 13, in units of pixel row.

Meanwhile, the reset line 121-2 supplies the gate electrode of the resettransistor 23 with the reset pulse RSTi of an i^(th) row, which isoutput from the vertical driving unit 13, in units of pixel row, andsupplies the gate electrode of the charge discharging transistor 26 ofthe same pixel row with the reset pulse RSTi of an i^(th) row in unitsof pixel row.

As described above, in the CMOS image sensor according to the secondembodiment, the charge discharging transistor 26 is driven using thereset pulse RST when performing the driving for reading the signals fromthe unit pixel 20A. Further, since the reset pulse RST is used as thecharge discharging pulse OFG of the charge discharging transistor 26,even if a signal line dedicated for the charge discharging pulse OFG isnot provided, the selection line 121-2 for the reset pulse RST is usedas the dedicated signal line, so that the charge discharging transistor26 can be driven.

That is, similarly to the first embodiment, in the CMOS image sensorhaving the pixel structure with the charge discharging transistor 26,even if the signal line dedicated for driving the charge dischargingtransistor 26 is not added, the charge discharging transistor 26 can bedriven. In this way, the configuration, in which the signal linededicated for driving the charge discharging transistor 26 is notnecessary, is employed, so that the reduction in the layout efficiencyor sensitivity accompanied by the addition of the charge dischargingtransistor 26 is not caused.

In addition, in the CMOS image sensor including the unit pixel 20A ofthe circuit example 1 as shown in FIG. 2, when the reset pulse RST andthe charge discharging pulse OFG are held in common, the chargedischarging transistor 26 operates only for charge discharging whenexposure is started. This is because, if the charge dischargingtransistor 26 is turned on and the reset pulse RST is in an active stateduring the reading period for which photoelectric charges are held inthe floating capacitance Cfd, the reset transistor 23 is also turned onand thus the accumulated charges of the floating capacitance Cfd may belost.

4. Third Embodiment

[System Configuration]

FIG. 9 is a circuit diagram illustrating the configuration of mainelements of the CMOS image sensor according to the third embodiment ofthe invention. In FIG. 9, the same reference numerals are used todesignate the same elements as those shown in FIG. 4.

The CMOS image sensor according to the embodiment is based on the CMOSimage sensor in which the unit pixel of the circuit example 2 shown inFIG. 3, that is, the unit pixel 20B including the memory unit 28 thatholds charges inside the pixel is two-dimensionally arranged. Further,in order to simplify FIG. 9, two unit pixels 20B-i and 20B-i-1 of acertain pixel column, which are vertically adjacent to each other, arerepresentatively shown.

As shown in FIG. 9, in the unit pixels 20B-i and 20B-i-1, as the pixeldriving lines 121, for example, four driving lines such as thetransmission line 121-1, the reset line 121-2, the selection line 121-3and the transmission line 121-4 are provided to each pixel row. Thesedriving lines 121-1, 121-2, 121-3 and 121-4 are respectively connectedto output terminals corresponding to the vertical driving unit 13 byeach pixel row.

Then, description will be given while focusing on the i^(th) pixel row.The transmission line 121-1 supplies the gate electrode of the transfertransistor 22 with the transmission pulse TRGi of the i^(th) row, whichis output from the vertical driving unit 13, in units of pixel row. Theselection line 121-3 supplies the gate electrode of the selecttransistor 25 with the selection pulse SELi of the i^(th) row, which isoutput from the vertical driving unit 13, in units of pixel row. Thetransmission line 121-4 supplies the gate electrode of the memory unit28 with the transmission pulse TRXi of the i^(th) row, which is outputfrom the vertical driving unit 13, in units of pixel row.

Meanwhile, the reset line 121-2 supplies the gate electrode of the resettransistor 23 with the reset pulse RSTi of the i^(th) row, which isoutput from the vertical driving unit 13, in units of pixel row, andsupplies the gate electrode of the charge discharging transistor 26 ofthe previous row (i.e., the (i−1)^(th) row) with the reset pulse RSTi ofthe i^(th) row in units of pixel row. That is, as the charge dischargingpulse OFGi-1 of the pixel 20B-i-1 for which signal reading is previouslyperformed with respect to the row scanning direction, the reset pulseRSTi of the pixel 20B-i, for which signal reading is performedthereafter, is commonly used.

If the pixel structure including the memory unit 28 according to thecircuit example 2 is employed, it is possible to obtain the effect ofpreventing overflowing of charges upon the saturation. In the CMOS imagesensor including the unit pixel 20B according to the circuit example 2,as well known in the related art, photoelectric charges accumulated inthe photodiode 21 through the global exposure are simultaneouslytransferred to the memory unit 28 with respect to all pixels, and heldin the capacitance Cm. Then, the photoelectric charges are transferredfrom the FD section 27 to the memory unit 28 upon the reading.

[Driving Example]

Next, one example of a driving method of the CMOS image sensor accordingto the third embodiment will be described using the timing chart of FIG.10.

First, with respect to all pixel rows, the reset pulse RSTi, thetransmission pulse TRXi and the transmission pulse TRXi are in an activestate at time t21, and charges accumulated in the photodiode 21 aredischarged, so that the global exposure is started. At this time, whenthe charges accumulated in the photodiode 21 are completely dischargedvia the charge discharging transistor 26, transfer driving to the memoryunit 28 by the transmission pulse TRXi may be omitted.

During the period for which the global exposure is performed,photoelectric charges are accumulated in the photodiode 21. Then, thetransmission pulse TRXi is in the active state at time t22 at which theglobal exposure is ended, so that the charges accumulated in thephotodiode 21 are transferred to the memory unit 28 and held in thecapacitance Cm. The above operation is performed during the period forwhich the global exposure is performed.

Reading of signals from the unit pixel 20B is performed by each pixelrow. In detail, if the selection pulse SELi-1 is in the active state attime t23, the signal reading period of the (i−1)^(th) row is reached. Inthe signal reading period of the (i−1)^(th) row, first, the reset pulseRSTi-1 is in the active state at time t24, so that the floatingcapacitance Cfd of the FD section 27 is reset. Then, the voltage of theFD section 27 at that time is read as the reset level (P period).

Thereafter, the transmission pulse TRGi-1 is in the active state at timet25, so that the photoelectric charges are transferred to thecapacitance Cm of the memory unit 28 to the floating capacitance Cfd ofthe FD section 27. Then, the photoelectric charges held in the floatingcapacitance Cfd is read as the signal level (D period).

Consecutively, the reason for entering the reading operation of thei^(th) row will be described. An operation is performed between thereading operation of the (i−1)^(th) row and the reading operation of thei^(th) row to prevent the photoelectric charges from overflowing due tothe saturation of the photodiode 21. In detail, in the period betweent26 and t27, the reset pulse RSTi is in the active state and the chargedischarging transistor 26 is turned on with a desired frequency, so thatthe photoelectric charges of the photodiode 21 are charged to the levelof the supply voltage Vdd. Thus, the saturation of the photodiode 21 canbe prevented.

As described above, similarly to the second embodiment, the CMOS imagesensor according to the third embodiment employs a configuration inwhich the signal line dedicated for driving the charge dischargingtransistor 26 is not necessary. Consequently, similarly to the first andsecond embodiments, the reduction in the layout efficiency orsensitivity accompanied by the addition of the charge dischargingtransistor 26 is not caused.

FIG. 11 illustrates a driving example in the case of the rolling shutteroperation. Before the floating capacitance Cfd of the FD section 27 isreset in the reading period, the transmission pulse TRXi-1 is in anactive state and the charges accumulated in the photodiode 21 aretransferred to the memory unit 28, so that the exposure period is ended.Then, after the reset pulse RSTi-1 in the active state and the floatingcapacitance Cfd of the FD section 27 is reset, the reset level is read(P period). Consecutively, after the transmission pulse TRGi-1 is in anactive state and the photoelectric charges are transferred from thememory unit 28 to the FD section 27, the signal level is read (Dperiod).

In the signal reading period, before the floating capacitance Cfd isreset by the reset transistor 23, the charges accumulated in thephotodiode 21 are transferred to the capacitance Cm of the memory unit28. That is, the transmission pulse TRX is in the active state prior tothe reset pulse RST. In this way, the charges accumulated in thephotodiode 21 can be evacuated to the memory unit 28, so that it ispossible to realize a driving method for preventing the accumulatedcharged from being lost from the photodiode 21 through driving by thenext reset pulse RST.

When the driving example is employed, the reset pulse RST and the chargedischarging pulse OFG of the same pixel can be held in common.Otherwise, as described in the embodiment, with respect to the rowscanning direction, as a line of the charge discharging pulse OFG of apixel which is previously read, the reset line 121-2 of the reset pulseRST of a pixel, which is read thereafter, may be held in common.

5. Fourth Embodiment

FIG. 12 is a circuit diagram illustrating the configuration of mainelements of the CMOS image sensor according to the fourth embodiment ofthe invention. In FIG. 12, the same reference numerals are used todesignate the same elements as those shown in FIG. 8.

Similarly to the second embodiment, the CMOS image sensor according tothe embodiment is based on the CMOS image sensor in which the unit pixel20A of the circuit example 1 shown in FIG. 2 is two-dimensionallyarranged. Further, in order to simplify FIG. 12, two unit pixels 20A-iand 20A-i-1 of a certain pixel column, which are vertically adjacent toeach other, are representatively shown.

As shown in FIG. 12, in the unit pixels 20A-i and 20A-i-1, as the pixeldriving lines 121, for example, three driving lines such as thetransmission line 121-1, the reset line 121-2 and the selection line121-3 are provided to each pixel row. These driving lines 121-1, 121-2and 121-3 are respectively connected to output terminals correspondingto the vertical driving unit 13 by each pixel row.

Then, description will be given while focusing on the i^(th) pixel row.The transmission line 121-1 supplies the gate electrode of the transfertransistor 22 with the transmission pulse TRGi of the i^(th) row, whichis output from the vertical driving unit 13, in units of pixel row, andsupplies the gate electrode of the charge discharging transistor 26 withthe previous row (i.e., the (i−1)^(th) row). That is, as the chargedischarging pulse OFGi-1 of the charge discharging transistor 26 of thepixel 20A-i-1 for which signal reading is previously performed withrespect to the row scanning direction, the transmission pulse TRGi ofthe pixel 20A-i, for which signal reading is performed thereafter, iscommonly used.

The reset line 121-2 supplies the gate electrode of the reset transistor23 with the reset pulse RSTi of the i^(th) row, which is output from thevertical driving unit 13, in units of pixel row. The selection line121-3 supplies the gate electrode of the select transistor 25 with theselection pulse SELi of the i^(th) row, which is output from thevertical driving unit 13, in units of pixel row.

As described above, in the CMOS image sensor according to the fourthembodiment, the charge discharging transistor 26 is driven using thetransmission pulse TRGi when performing the driving for reading thesignals from the unit pixel 20A. Further, since the transmission pulseTRGi is used as the charge discharging pulse OFG, even if a signal linededicated for the charge discharging pulse OFG is not provided, theselection line 121-1 for the transmission pulse TRGi is used as thededicated signal line, so that the charge discharging transistor 26 canbe driven.

That is, similarly to the first to third embodiments, even if the signalline dedicated for driving the charge discharging transistor 26 is notadded, the charge discharging transistor 26 can be driven. In this way,the configuration, in which the signal line dedicated for driving thecharge discharging transistor 26 is not necessary, is employed, so thatthe reduction in the layout efficiency or sensitivity accompanied by theaddition of the charge discharging transistor 26 is not caused.

In particular, as the charge discharging pulse OFGi-1 of the chargedischarging transistor 26 of the unit pixel 20A-i-1 for which signalreading is previously performed with respect to the row scanningdirection, the transmission pulse TRGi of the unit pixel 20A-i, forwhich signal reading is performed thereafter, is commonly used, so thatthe following effects can be obtained. That is, before the chargesaccumulated in the photodiode 21 are transferred to the FD section 27,the charge discharging transistor 26 is driven, so that it is possibleto prevent driving in which the charges accumulated in the photodiode 21may be lost.

6. Fifth Embodiment

FIG. 13 is a circuit diagram illustrating the configuration of mainelements of the CMOS image sensor according to the fifth embodiment ofthe invention. In FIG. 13, the same reference numerals are used todesignate the same elements as those shown in FIG. 9.

The CMOS image sensor according to the embodiment is based on the CMOSimage sensor in which the unit pixel of the circuit example 2 shown inFIG. 3, that is, the unit pixel 20B including the memory unit 28 thatholds charges inside the pixel is two-dimensionally arranged. Further,in order to simplify FIG. 13, two unit pixels 20B-i and 20B-i-1 of acertain pixel column, which are vertically adjacent to each other, arerepresentatively shown.

As shown in FIG. 13, in the unit pixels 20B-i and 20B-i-1, as the pixeldriving lines 121, for example, four driving lines such as thetransmission line 121-1, the reset line 121-2, the selection line 121-3and the transmission line 121-4 are provided to each pixel row. Thesedriving lines 121-1, 121-2, 121-3 and 121-4 are respectively connectedto output terminals corresponding to the vertical driving unit 13 byeach pixel row.

Then, description will be given while focusing on the i^(th) pixel row.The transmission line 121-1 supplies the gate electrode of the transfertransistor 22 with the transmission pulse TRGi of the i^(th) row, whichis output from the vertical driving unit 13, in units of pixel row. Thereset line 121-2 supplies the gate electrode of the reset transistor 23with the reset pulse RSTi of the i^(th) row, which is output from thevertical driving unit 13, in units of pixel row. The selection line121-3 supplies the gate electrode of the select transistor 25 with theselection pulse SELi of the i^(th) row, which is output from thevertical driving unit 13, in units of pixel row.

The transmission line 121-4 supplies the gate electrode of the memoryunit 28 with the transmission pulse TRXi of the i^(th) row, which isoutput from the vertical driving unit 13, in units of pixel row, andsupplies the gate electrode of the charge discharging transistor 26 ofthe previous row (i.e., the (i−1)^(th) row) with the transmission pulseTRXi of the i^(th) row in units of pixel row. That is, as the chargedischarging pulse OFGi-1 of the pixel 20B-i-1 for which signal readingis previously performed with respect to the row scanning direction, thetransmission pulse TRXi of the pixel 20B-i, for which signal reading isperformed thereafter, is commonly used.

As described above, in the CMOS image sensor according to the fifthembodiment, the charge discharging transistor 26 is driven using thetransmission pulse TRXi when performing the driving for reading thesignals from the unit pixel 20B. Further, since the transmission pulseTRXi is used as the charge discharging pulse OFG, even if a signal linededicated for the charge discharging pulse OFG is not provided, thetransmission line 121-4 for the transmission pulse TRXi is used as thededicated signal line, so that the charge discharging transistor 26 canbe driven.

That is, similarly to the first to fourth embodiments, even if thesignal line dedicated for driving the charge discharging transistor 26is not added, the charge discharging transistor 26 can be driven. Inthis way, the configuration, in which the signal line dedicated fordriving the charge discharging transistor 26 is not necessary, isemployed, so that the reduction in the layout efficiency or sensitivityaccompanied by the addition of the charge discharging transistor 26 isnot caused.

Further, as the charge discharging pulse OFGi-1 of the chargedischarging transistor 26 of the pixel 20B-i-1 for which signal readingis previously performed with respect to the row scanning direction, thetransmission pulse TRXi of the pixel 20B-i, for which signal reading isperformed thereafter, is commonly used, so that the effects identical tothat obtained in the fourth embodiment can be obtained. That is, beforethe charges accumulated in the photodiode 21 are transferred to the FDsection 27, the charge discharging transistor 26 is driven, so that itis possible to prevent driving in which the charges accumulated in thephotodiode 21 may be lost.

In the fourth and fifth embodiments, in the state in which the chargesare held in the floating capacitance Cfd of the FD section 27 or thecapacitance Cm of the memory unit 28, when it is necessary to extractthe charges accumulated in the photodiode 21, the CMOS image sensor isdesigned as follows. That is, as shown in FIG. 14, it is necessary todesign the CMOS image sensor such that the potential barrier of thecharge discharging transistor 26 is lower than the potential barrier ofthe transfer transistor 22.

When the potential of the elements is designed as described above, sincecharges may not be transferred to all pixels at the same time, therolling shutter operation, in which the exposure period is sequentiallyended along the row scanning direction, is performed. However, since itis not limited to signal reading in a typical rolling shutter operation,it is possible to obtain a high quality image having highsynchronization and low distortion. The charge transfer is performed inthe row scanning direction, so that the charges can be transferred tothe floating capacitance Cfd of the FD section 27 without dischargingthe charges accumulated in the photodiode 21 as shown in FIG. 15.

Further, when retention charges exist in the floating capacitance Cfd ofthe FD section 27, if driving is simultaneously performed with respectto all pixels by the transmission pulse TRG, since excess charges aredischarged from a side of the charge discharging transistor 26 having alow potential barrier as shown in FIG. 16A, the excess charges are notadded to the retention charges and the discharging of the excess chargesis completed as shown in FIG. 16B.

7. Application

In the above embodiments, the CMOS image sensor in which the unit pixel20 is two-dimensionally arranged has been described as an example.However, the invention is not limited thereto. In detail, in the case ofthe circuit example 2 as shown in FIG. 3, that is, the unit pixel 20Bincluding the memory unit 28 inside the pixel, the invention can beapplied to a CMOS image sensor having a holding structure by pluralpixels, in which a part of elements originally provided to each pixel isheld in common among a plurality of pixels.

[Pixel Structure for Holding by Two Horizontal Pixels]

FIG. 17 is a circuit diagram illustrating the configuration of mainelements when the invention is applied to a pixel structure in which apart of elements is held in common between two pixels adjacent to eachother in the horizontal direction (transverse direction). In FIG. 17,the same reference numerals are used to designate the same elements asthose shown in FIG. 9.

As shown in FIG. 17, in the pixel holding structure according to theapplication, the reset transistor 23, the amplifying transistor 24, theselect transistor 25 and the FD section 27 are held in common betweenthe two pixels adjacent to each other in the horizontal direction. Withrespect to the pixel structure for holding by two horizontal pixels,according to the application, the selection pulse SELi when performingthe driving for reading signals from the unit pixel 20B is used as thecharge discharging pulse OFG.

In detail, referring to FIG. 17, as the pixel driving lines 121, forexample, four driving lines such as the transmission line 121-1, thereset line 121-2, the selection line 121-3 and the transmission line121-4 are provided. Then, description will be given while focusing onthe i^(th) pixel row. The transmission line 121-1 supplies the gateelectrode of the transfer transistor 22 with the transmission pulse TRGiof the i^(th) row, which is output from the vertical driving unit 13, inunits of pixel row. The reset line 121-2 supplies the gate electrode ofthe reset transistor 23 with the reset pulse RSTi of the i^(th) row,which is output from the vertical driving unit 13, in units of pixelrow.

The selection line 121-3 supplies the gate electrode of the selecttransistor 25 with the selection pulse SELi of the i^(th) row, which isoutput from the vertical driving unit 13, in units of pixel row, andsupplies the gate electrode of the charge discharging transistor 26 ofthe previous row (i.e., the (i−1)^(th) row) with the selection pulseSELi of the i^(th) row in units of pixel row. In this way, as the chargedischarging pulse OFGi-1 of the pixel 20B-i-1 for which signal readingis previously performed with respect to the row scanning direction, theselection pulse SELi of the select transistor 25 of the pixel 20B-i, forwhich signal reading is performed thereafter, is commonly used.

As described above, the invention can also be applied to the pixelstructure for holding by the two horizontal pixels. Herein, theselection pulse SELi is used as the charge discharging pulse OFG, butthe invention is not limited thereto. That is, the reset pulse RST canbe used as the charge discharging pulse OFG similarly to the thirdembodiment, the transmission pulse TRG can be used as the chargedischarging pulse OFG similarly to the fourth embodiment, or thetransmission pulse TRX can be used as the charge discharging pulse OFGsimilarly to the fifth embodiment.

In a pixel holding structure to which the invention is applied, elementsheld in common are not limited to all elements located after the FDsection 27. Further, a pixel holding structure is not limited to holdingby pixels arranged in the horizontal direction. That is, the pixelholding structure may include a holding structure in which elements areheld in common between two pixels adjacent to each other in the verticaldirection (longitudinal direction), a holding structure in whichelements are held in common among four pixels adjacent to each other inthe horizontal and vertical directions, or the like. In brief, variousconfigurations may be employed if the driving methods according to theprevious embodiments can be realized.

8. Modification

According to the previous embodiments, the case in which the unit pixel20 (20A and 20B) has the pixel configuration including the fourtransistors in addition to the charge discharging transistor 26 has beendescribed as an example. However, the unit pixel 20 is not limited tothe pixel configuration including the four transistors. In detail,differently from the embodiment in which the selection pulse SEL is usedas the charge discharging pulse OFG, for example, it is possible toemploy a pixel configuration, which includes three transistors, exceptfor the select transistor 25, and, instead, the amplifying transistor 24has the pixel selection function of the select transistor 25.

Further, according to the previous embodiments, the invention is appliedto the CMOS image sensor including the unit pixel which istwo-dimensionally arranged to detect signal charges corresponding to theamount of visible rays as a physical quantity. However, application ofthe invention is not limited to the CMOS image sensor. That is, theinvention can be applied to the whole of an X-Y address type solid-stateimage capturing apparatus having an electronic shutter function.

Further, application of the invention is not limited to a solid-stateimage capturing apparatus that captures an image by detecting thedistribution of the amount of an incident visible ray. That is, theinvention can be applied to a solid-state image capturing apparatus thatcaptures the distribution of the amount of incident infrared rays or Xrays, or the incident amount of grains or the like as an image. Inaddition, in a broad sense, a physical quantity distribution detectionapparatus (e.g., a fingerprint detection sensor or the like), whichcaptures an image by detecting the distribution of other physicalquantities such as pressures or electrostatic capacitances, may beemployed as a solid-state image capturing apparatus.

Moreover, a solid-state image capturing apparatus may be prepared in theform of one chip, or a module having an image capturing function, whichis formed by packaging an image capturing unit, a signal processing unitand an optical system.

9. Electronic Apparatus

Application of the invention is not limited to a solid-state imagecapturing apparatus. That is, the invention can be applied to anelectronic apparatus, which uses a solid-state image capturing apparatusin an image capturing unit (photoelectric conversion unit), such as animage capturing apparatus (e.g., a digital still camera, a video cameraor the like), a portable terminal (e.g., a portable phone or the like)having an image capturing function, or a copy machine which uses asolid-state image capturing apparatus in an image reading unit. Further,the modular type mounted on an electronic apparatus, that is, a cameramodule may be employed as an image capturing apparatus.

[Image Capturing Apparatus]

FIG. 18 is a block diagram illustrating a configuration example of animage capturing apparatus which is one example of the electronicapparatus according to the invention. As shown in FIG. 18, the imagecapturing apparatus 100 according to the invention includes an opticalsystem including a lens group 101 or the like, an image capturingelement 102, a DSP circuit 103 serving as a camera signal processingunit, a frame memory 104, a display device 105, a recording device 106,an operating system 107, a power supply system 108 or the like. The DSPcircuit 103, the frame memory 104, the display device 105, the recordingdevice 106, the operating system 107 and the power supply system 108 areconnected to each other through a bus line 109.

The lens group 101 captures incident light (image light) from an objectand allows an image to be formed on an image capturing surface of theimage capturing element 102. The image capturing element 102 convertsthe amount of the incident light, which is formed on the image capturingsurface as the image by the lens group 101, into an electric signal inunits of pixels, and outputs a pixel signal. As the image capturingelement 102, the CMOS image sensor according to the first to fifthembodiments or the application are used.

The display device 105 includes a panel type display device, such as aliquid crystal display device or an organic EL (Electro Luminescence)display device, and displays a dynamic image or a still image capturedby the image capturing element 102. The recording device 106 records thedynamic image or the still image, which has been captured by the imagecapturing element 102, on a recording medium such as a video tape or aDVD (Digital Versatile Disk).

The operating system 107 issues operation orders with respect to variousfunctions of the image capturing apparatus under the operation of auser. The power supply system 108 various types of power to the DSPcircuit 103, the frame memory 104, the display device 105, the recordingdevice 106 and the operating system 107 as operating power.

The image capturing apparatus 100 is applied to a video camera or adigital still camera, and, further, to camera module used for mobileequipment such as a portable phone. In the image capturing apparatus100, the CMOS image sensor according to the first to fifth embodimentsor the application can be used as the image capturing element 102.

It should be understood by those skilled in the art that variousmodifications, combinations, sub-combinations and alterations may occurdepending on design requirements and other factors insofar as they arewithin the scope of the appended claims or the equivalents thereof.

1. A solid-state image capturing apparatus comprising: a pixel arrayunit including two-dimensionally arranged pixels each including aphotoelectric conversion unit, a transfer transistor that transferscharges accumulated in the photoelectric conversion unit, and a chargedischarging transistor that selectively discharges the chargesaccumulated in the photoelectric conversion unit; and a driving unitthat performs driving for reading signals from each pixel of the pixelarray unit, and drives the charge discharging transistor by using asignal for driving the reading.
 2. The solid-state image capturingapparatus according to claim 1, wherein the pixel includes a selecttransistor that performs pixel selection, and the driving unit drivesthe charge discharging transistor by using a signal used for driving theselect transistor.
 3. The solid-state image capturing apparatusaccording to claim 2, wherein the pixel includes: a floating capacitancethat holds the charges transferred from the photoelectric conversionunit by the transfer transistor; and a memory unit that holds theaccumulated charges, separately from the floating capacitance.
 4. Thesolid-state image capturing apparatus according to claim 2, wherein thedriving unit has a driving signal of the charge discharging transistorof a pixel, for which signal reading is previously performed in a rowscanning direction, together with a driving signal of the selecttransistor of a pixel for which signal reading is performed thereafter.5. The solid-state image capturing apparatus according to claim 1,wherein the pixel includes a reset transistor for resetting the pixel,and the driving unit drives the charge discharging transistor by using asignal used for driving the reset transistor.
 6. The solid-state imagecapturing apparatus according to claim 1, wherein the driving unitdrives the charge discharging transistor by using a signal used fordriving the transfer transistor.
 7. The solid-state image capturingapparatus according to claim 6, wherein the driving unit has a drivingsignal of the charge discharging transistor of a pixel, for which signalreading is previously performed in a row scanning direction, togetherwith a driving signal of the transfer transistor of a pixel for whichsignal reading is performed thereafter.
 8. The solid-state imagecapturing apparatus according to claim 1, wherein the pixel includes: afloating capacitance that holds the charges transferred from thephotoelectric conversion unit by the transfer transistor; and a memoryunit that holds the accumulated charges, separately from the floatingcapacitance, wherein the driving unit drives the charge dischargingtransistor by using a signal used for transferring the chargesaccumulated in the photoelectric conversion unit to a memory unit. 9.The solid-state image capturing apparatus according to claim 1, whereinthe driving unit drives the charge discharging transistor by using asignal used for driving the transfer transistor.
 10. The solid-stateimage capturing apparatus according to claim 9, wherein the driving unithas a driving signal of the charge discharging transistor of a pixel,for which signal reading is previously performed in a row scanningdirection, together with a driving signal of the transfer transistor ofa pixel for which signal reading is performed thereafter.
 11. Thesolid-state image capturing apparatus according to claim 1, wherein thepixel includes: a floating capacitance that holds the chargestransferred from the photoelectric conversion unit by the transfertransistor; and a memory unit that holds the accumulated charges,separately from the floating capacitance, wherein, the driving unitdrives the charge discharging transistor by using a signal used fortransferring the charges accumulated in the photoelectric conversionunit to a memory unit.
 12. The solid-state image capturing apparatusaccording to claim 11, wherein the driving unit has a driving signal ofthe charge discharging transistor of a pixel, for which signal readingis previously performed in a row scanning direction, together with asignal transferred to the memory unit of a pixel for which signalreading is performed thereafter.
 13. The solid-state image capturingapparatus according to claim 9 or 13, wherein a potential barrier of thecharge discharging transistor is lower than a potential barrier of thetransfer transistor with respect to the charges accumulated in thephotoelectric conversion unit.
 14. An electronic apparatus comprising asolid-state image capturing apparatus, the solid-state image capturingapparatus including: a pixel array unit including two-dimensionallyarranged pixels each including a photoelectric conversion unit, atransfer transistor that transfers charges accumulated in thephotoelectric conversion unit, and a charge discharging transistor thatselectively discharges the charges accumulated in the photoelectricconversion unit; and a driving unit that performs driving for readingsignals from each pixel of the pixel array unit, and drives the chargedischarging transistor by using a signal for driving the reading.